Thick-film resistors are employed in hybrid electronic circuits to provide a wide range of resistor values. Such resistors are printed on ceramic substrates using thick-film pastes, or inks, which are typically composed of an organic vehicle, a glass frit composition, and an electrically-conductive material. After printing, thick-film inks are typically dried and then sintered, or fired, to convert the ink into a solid film that adheres to the ceramic substrate. During firing, the ink is heated at a rate that is sufficiently slow to allow the organic vehicle of the ink to burn off, which generally begins at about 345.degree. C. and is completed at about 400.degree. C. to 450.degree. C. with commercially available ink compositions. Peak firing temperatures are typically in the range of about 850.degree. C. to 950.degree. C. Both physical and chemical changes occur within the thick film during sintering, by which the conduction network or microstructure of the resistor is formed. Various additives may be used to achieve specific desired resistivity, stability and temperature characteristics.
Ruthenium-based resistors are widely recognized in the art for their reliability and stable resistance values. A limitation to ruthenium-based thick-film resistors is that their inks must be fired in oxidizing atmospheres in order to prevent the ruthenium compound, usually ruthenium dioxide (RuO2), from being reduced to metallic ruthenium. It has been reported that reduction of ruthenium dioxide begins at about 350.degree. C. in a nitrogen atmosphere.
Thick-film conductors for hybrid circuits are also formed using thick-film inks, with thick-film copper conductors being widely used in view of their low bulk resistivity (sheet resistance about 3 milliohms per square). Thick-film copper inks are fired in a nitrogen atmosphere to avoid the metallic copper from being oxidized into copper oxide, which would prevent the resulting conductor from having high conductivity (low resistivity) and adequate solderability.
From the above, one can see that thick-film ruthenium-based resistors and copper conductors have conflicting processing requirements --ruthenium-based resistors require an oxidizing firing environment, while copper conductors require a non-oxidizing environment. Various solutions have been proposed to overcome this limitation and permit the simultaneous use of ruthenium-based resistors and copper conductors on the same hybrid circuit board. One solution is a process taught by Kuo, Thick Film Copper Conductor and Ruthenium-Based Resistor System for Resistor Circuits, The International Journal for Hybrid Microelectronics, International Microelectronics Symposium (1983), that requires a first firing in air at 850.degree. C. to 950.degree. C. for the thick-film copper conductor, a second firing in air for the ruthenium-based resistor, and then firing at about 260.degree. C. to 400.degree. C. in a hydrogen-nitrogen atmosphere to reduce the oxidized copper produced when the copper was fired in air. The copper conductors and ruthenium-based resistors produced by this process are disclosed as having desirable electrical properties.
Another process-related solution is to print and then fire a ruthenium-based thick-film ink in air at 850.degree. C. to 950.degree. C., followed by printing and firing a thick- film copper conductor ink at 600.degree. C. in nitrogen. A significant drawback to this process is that the resulting resistors cannot be measured for resistance and temperature-related properties like TCR (temperature coefficient of resistance) until after the conductor had been printed and fired, resulting in scrappage that could be otherwise avoided.
Other suggested solutions have required composition changes to the ruthenium-based thick-film ink. One such solution taught by Hankey et al., Introduction of a Novel Copper Compatible Nitrogen Firing Resistor System, IMC Proceedings (1986), p. 98-102, entails incorporating ruthenium dioxide in a perovskite structure to provide stability during firing in nitrogen. However, doing so significantly complicates the formulation process for obtaining a thick-film resistor of desired resistance value. Another alternative is to forego the advantages of ruthenium-based thick-film resistors, and instead employ base metal thick-film inks that can be fired in a nitrogen atmosphere so as to be compatible with copper conductors. Base metal (non-noble metal) base resistors are not as stable as ruthenium-based resistors, and generally require glass passivation to promote their stability.
From the above, it can be seen that present practices involving the processing of thick-film ruthenium-based resistors with copper conductors are generally complicated. Again, the incompatibility arises from the conventional wisdom that thick-film ruthenium-based resistors must be fired in an atmosphere that will adversely oxidize copper conductors. From the standpoint of cost and stability, it would be highly desirable if a less complicated process was available that enabled the production of thick-film ruthenium-based resistors with copper conductors.